1. Field of the Invention
The present invention relates to an electrode for fuel cell and a process for the production thereof.
2. Description of the Related Art
A solid polymer electrolyte type fuel cell (PEFC) is composed of as an electrolyte of a cation-exchange membrane which is a solid polymer electrolyte such as perfluorocarbonsulfonic acid membrane and an anode and cathode connected to the ion-exchange membrane on the respective side thereof. In operation, hydrogen is supplied into the anode while oxygen is supplied into the cathode so that an electrochemical reaction occurs to generate electricity. The electrochemical reaction occurring on these electrodes will be shown below.
Anode: H2xe2x86x922H++2exe2x88x92
Cathode: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
Total reaction: H2+1/2O2xe2x86x92H2O
As can be seen in these reaction formulae, the reaction on the electrodes proceed only on the three-phase boundary site where gas of the active materials, i.e., (hydrogen or oxygen), proton (H+) and electron (exe2x88x92) can be received and released at the same time.
An example of the electrode for fuel cell having such a function is a solid polymer electrolyte-catalyst composite electrode made of a cation-exchange resin as a solid polymer electrolyte, carbon particles and a catalyst metal. An example of the structure of a cation-exchange resin-catalyst composite electrode made of a cation-exchange resin as a solid polymer electrolyte, carbon particles supporting a catalyst metal showing a high catalytic activity for the reduction reaction of oxygen and the oxidation reaction of hydrogen is shown in FIG. 9. In FIG. 9, the reference numeral la indicates a carbon particle, the reference numeral 2a indicates a cation-exchange resin, the reference numeral 3a indicates an ion-exchange membrane, and the reference numeral 4a indicates a pore. As can be seen in FIG. 9, the carbon particles la supporting a catalyst metal and the cation-exchange resin 2a are three-dimensionally distributed and a plurality of pores 4a are formed in the porous electrode. The carbon as a support of catalyst metal forms an electron-conductive channel. The cation-exchange resin forms a proton-conductive channel. The pores form a channel for supplying oxygen or hydrogen and discharging water as a product. Further, these three channels are three-dimensionally spread in the electrode to form numerous three-phase boundaries at which gas, proton (H+) and electron (exe2x88x92) can be received and released at the same time, thereby providing a site for electrode reaction.
The electrode having such a structure has heretofore been prepared by a process including applying a paste made of a carbon particle supporting highly dispersed a catalyst metal particles such as platinum and suspension of PTFE (polytetrafluoroethylene) particles to a polymer film or electrically-conductive porous carbon substrate to form a film of the paste (normally to a thickness of from 3 xcexcm to 30 xcexcm), heating and drying the film, followed by applying a cation-exchange resin solution to the film so that the film is impregnated with the cation-exchange resin solution, and then drying the film or a process including applying a paste made of the foregoing carbon particle supporting catalyst, a cation-exchange resin solution and optionally PTFE particles to a polymer film or electrically-conductive porous carbon electrode substrate to form a film of the paste (normally to a thickness of from 3 xcexcm to 30 xcexcm), and then drying the film. As the cation-exchange resin solution there is used one obtained by dissolving a material having the same composition as the previously mentioned ion-exchange membrane in an organic solvent such as alcohol or a mixture of an organic solvent and water to form a liquid like solution. As the suspension of PTFE particles there is used a suspension of PTFE particles having a diameter of about 0.23 xcexcm.
PEFC is expensive. This prevents PEFC from being put in practical use. In particular, metals belonging to the platinum group which are used as catalyst are expensive. This is a major factor causing the rise of the cost of PEFC. Therefore, how the amount of platinum group metal as catalyst metal to be supported on the electrode can be reduced is the key to technical development in the art.
The conventional electrode used a catalyst metal particle belonging to a platinum group metal supported on carbon. The activity of the electrode depends greatly on the surface of the platinum group metal particle. Therefore, it is an ordinary practice to reduce the particle diameter of the platinum group metal and hence increase the surface area of the platinum group metal per unit weight thereof, enhancing the catalytic activity per unit weight of the platinum group metal. At present, carbon supporting a platinum group metal having an average particle diameter of about 4 nm is used as a catalyst metal. However, it is necessary to support a platinum group metal both on the cathode and anode in an amount as great as 0.4 mg/cm2 or more in order to obtain sufficient characteristics for practical use. Further, the conventional electrode prepared by the production processes described above shows a reduced percent utilization of the catalyst metal supported on carbon, e.g., only about 10%, further lowering the activity against the total electrode reactions (see Edson A. Tisianelli, xe2x80x9cJ. Electroanal. Chem.xe2x80x9d, 251, 275, 1988). This is attributed to the fact that the conventional production processes involve the mixing of carbon particles supporting a catalyst metal particle such as platinum supported thereon with a cation-exchange resin. In other words, the carbon particle as a support has a particle diameter as small as 30 nm for example. The carbon particle to be mixed with the cation-exchange resin solution is composed of aggregates of some carbon particles having considerably dense roughness formed on the surface thereof. On the other hand, the cation-exchange resin solution has a certain viscosity and thus cannot penetrate deep into the central portions of the aggregate of carbon particles even by a process including impregnating the dispersion film layer made of carbon particles and PTFE particles with a cation-exchange resin solution or a process including the use of a paste obtained by mixing carbon particles, PTFE particles and a cation-exchange resin solution. This phenomenon makes it impossible to form a three-phase boundary in the deep portion in the aggregate of carbon particles. Therefore, the catalyst metal particle positioned at this portion takes no part in electrode reaction, causing a drop of the percent utilization of the catalyst metal. The structure of such an electrode is shown in FIG. 10. As shown in FIG. 10, carbon particles 3b supporting catalyst particles 1b and 2b gather together to form an aggregate of carbon particles (4 piece of carbon particles constituting an aggregate in this figure). In this arrangement, since cation-exchange resin 4b doesn""t penetrate into a deep portion 5b of the aggregate of carbon particles, the catalyst particles 1b which are positioned on the contact area of carbon particle with the cation-exchange resin effectively act on the electrode reaction and a catalyst particle 2b which has no area in contact with the cation-exchange resin doesn""t effectively act on the electrode reaction.
In order to enhance the percent utilization of catalyst metal, studies have been made of supporting of a catalyst metal on the portion where the surface of carbon particle contacts a cation-exchange resin (the state shown in FIG. 10 excluding the catalyst particles 2b). However, the mere study of how the carbon supporting catalyst and the cation-exchange resin are three-dimensionally arranged in the electrode on the basis of the conventional macroscopic consideration of the structure of three-phase boundary in the electrode is limited sufficient for the drastic enhancement of the percent utilization of catalyst metal. More detailed approach from the micro structure of three-phase boundary in the electrode has been requested to make its drastic improvement.
As reported in, e.g., H. L. Yeager et al., xe2x80x9cJ. Electrochem. Soc.,xe2x80x9d 128, 1880, 1981, and Kokumi et al, xe2x80x9cJ. Electrochem. Soc.,xe2x80x9d 132, 2601, 1985, the microscopic observation of the structure of a cation-exchange resin shows that a cation-exchange resin has a proton-conductive passage, called as a cluster formed by a hydrophilic exchange functional group and its counter ions with water, and a hydrophobic backbone moiety made of Teflon, etc. Considering deeply these aspects, a gas (hydrogen or oxygen) as an active reaction material and water as a product of the cathode, not to mention proton, must transport through the proton-conductive passage but not through the hydrophobic backbone. Therefore, the inventor concluded from the microscopic further-considerations of electrode reaction that the three-phase boundary on which the reaction of electrode for fuel cell proceeds exists only on the site where the surface of the carbon particles contacts the proton-conductive passage in the cation-exchange resin. It means that the positional relationship of the catalyst metal with the proton-conductive passage in the cation-exchange resin and the distribution of the catalyst metal in the proton-conductive passage need to be more studied. In other words, the conventional processes for the production of an electrode for fuel cell, as previously mentioned, involving the mixing of carbon particles supporting a catalyst metal particle such as platinum supported thereon with a cation-exchange resin solution is considered to be inefficient method with the poor expectation that the catalyst metal particle such as platinum supported on carbon particles happens to contact the proton-conductive passage in the cation-exchange resin, resulting in the lower utilization of catalyst metal.
FIG. 11 is diagram illustrating the surface layer of a carbon particle in contact with the cation-exchange resin in the conventional electrode (what is further magnified diagram of the portion of FIG. 10 where the carbon particle contacts the carbon-exchanging resin). In FIG. 11, the reference numeral 1 indicates a carbon particle, the reference numeral 2 indicates a proton-conductive passage called a cluster of cation-exchange resin, the reference numeral 3 indicates the Teflon backbone of the cation-exchange resin, and the reference numerals 4 and 5 indicate a catalyst metal particle. As shown in FIG. 11 for example, the surface of the carbon particle 1 is covered with a cation-exchange resin composed of a proton-conductive passage 2 and a fluorocarbon backbone 3. The catalyst metal particles 4 and 5 are supported on the surface of the carbon particle 1. The catalyst metal particle 5 is located at the site of the proton-conductive passage 2 in the cation-exchange resin and thus can effectively act as a catalyst. On the contrary, the catalyst metal particle 4 is located at the site of the Teflon backbone 3 in the cation-exchange resin and thus cannot effectively act as a catalyst. Further, the region Z has a three-phase boundary formed therein but has no catalyst metal particle and this site takes no part in the reaction. In other words, in this kind of an electrode, the presence of the catalyst metal particle 4 causes a drop of the percent utilization of catalyst metal and the presence of the three-phase boundary Z causes a drop of the activity of the electrode.
Another problem with the conventional electrode for fuel cell will be described hereinafter. Examples of the fuel cell include those using hydrogen and methanol as a fuel. In the case where hydrogen is used as a fuel, methanol is stored on board taking into account the utility. Using a reformer utilizing the chemical reaction of methanol with water, methanol is converted to hydrogen in a required amount. The hydrogen thus produced is then supplied into PEFC. PEFC used for this purpose is a fuel cell using reformed fuel gas of methanol. On the other hand, in the latter case where methanol is used as a fuel, methanol is directly supplied into PEFC where it is direct electrochemically oxidized. PEFC used for this purpose is a direct methanol fuel cell (DMFC: Direct Methanol Fuel Cell). In these cases where such fuels are used, unlike in the case where pure hydrogen is used, anode is required to exhibit a high CO tolerance or high methanol oxidation properties. However, mere use of platinum cannot provide such properties. It is known that an activity which cannot be obtained by the use of a single platinum group metal element, e.g., CO tolerance can be obtained by alloying two or more metal elements, including platinum group metal element. Therefore, a platinum-ruthenium alloy is supported on the anode for PEFC using reformed fuel gas of methanol fuel cell. This attempt turns to good results when the amount of an alloy of two or more metals, including platinum group metal element, supported on carbon particles is large. However, when it is attempted to reduce the amount of an alloy of two or more metals, including platinum group metal element, supported on carbon particles for the purpose of lowering PEFC cost, this attempt does not necessarily turn to good results. This is because even when it is attempted to allow carbon particles having a remarkably large surface area per unit weight to be impregnated simultaneously or sequentially with two or more catalyst metals, including platinum group metal element, so that these metal elements are alloyed, the two or more metal elements, including platinum group metal element, are each independently dispersed to a high extent, drastically lowering the percent alloying. Thus, the desired CO tolerance and activity for the electrochemical oxidation reaction of methanol are deteriorated.
It is an object of the present invention to enhance the percent utilization of catalyst in an electrode for fuel cell and improve the properties thereof by improving the microscopic structure of three-phase boundary in the electrode and the structure of catalyst metal particles itself.
An electrode for fuel cell according to the invention comprises a solid polymer electrolyte-catalyst composite electrode containing a cation-exchange resin, carbon particles and a catalyst metal, the catalyst metal being loaded mainly on the site where the surface of the carbon particles contacts the proton-conductive passage in the cation-exchange resin. Preferably, the catalyst metal has a nucleus containing a metal (X) and a shell containing a metal (Y) which is not contained in the nucleus.
A process for the production of an electrode for fuel cell comprising a first step (absorption step) of absorbing a cation of a catalyst metal element into a cation-exchange resin in a mixture of its resin and carbon particles by ion-exchange reaction between a counter ion of the resin and the cations, and a second step (reduction step) of chemically reducing the cations in the mixture obtained at the first step. Preferably, a process for the production of a n electrode comprising, a first step of absorbing a cation of a catalyst metal element (Xxe2x80x2) into a cation-exchange resin in a mixture of its resin and carbon particles by ion-exchange reaction between a counter ion of the resin and the cation, and a second step of chemically reducing the cations in the mixture obtained at the first step, and a third step of absorbing a cation of a catalyst metal element (Yxe2x80x2) into the resin by ion-exchange reaction between the counter ion and the cation, and a forth step of chemically reducing the cation in the mixture obtained at the third step, wherein the element (Yxe2x80x2) is different from the element (Xxe2x80x2).
In the solid polymer electrolyte-catalyst composite electrode for fuel cell obtained by the production process of the invention, the catalyst metal is preferentially supported on the site where the surface of carbon particles contacts the proton-conductive passage in the cation-exchange resin. Thus, the amount of catalyst metal supported on the site accounts for 50 wt % or more of the total supported amount of catalyst metal.